The long-term goal of this investigator is to elucidate the epidemiology of St Louis encephalitis (SLE) virus. This particular project will focus on three questions: (1) to determine if this virus overwinters in Texas in its vector Culex quinquefaciatus, (2) to determine if the epidemic-causing potential of different variants of this virus correlates with differences in the amino acid sequence of the envelope protein, and (3) to assess dispersal patterns of different variants of this virus. The study area is Harris County, Texas, where the Harris County Mosquito Control Division (HCMCD) has maintained a very effective program of virus monitoring and isolation for more than a decade. HCMCD staff will be important collaborators in this project. The overwintering mechanism will be assessed by collecting mosquitoes using CDC light traps and with mechanical aspirators from resting sites under houses. Both methods have been demonstrated to be very effective. Viruses will be isolated from collected mosquitoes using procedures that the HCMCD staff have used effectively for many years. Virus variation will be assessed primarily by SSCP of a 750 bp region of the envelope protein gene, with some sequencing of selected isolates. Epidemic-causing potential of this virus will be determined by titre and duration of viremia in chickens. Patterns of distribution of virus variants will be determined by SSCP.